The present invention relates to a magnetic field inhomogeneity measurement method and apparatus, a phase correction method and apparatus and a magnetic resonance imaging apparatus, and more particularly to a method and apparatus for measuring magnetic field inhomogeneity in a space to be imaged based on the phase of pixel data of an image obtained by magnetic resonance imaging, a method and apparatus for correcting the phase of pixel data based on the measured magnetic field inhomogeneity, and a magnetic resonance imaging apparatus for obtaining images with water and fat separated based on the phase-corrected pixel data.
A magnetic resonance imaging apparatus is configured to generate a static magnetic field in a space containing a subject to be imaged, generate gradient and high frequency magnetic fields in the static magnetic field space, and produce (i.e., reconstruct) an image based on magnetic resonance signals generated from the magnetic field space. Since the magnetic resonance signals from fat have a different frequency than the magnetic resonance signals from water due to chemical shift, water and fat can be separately imaged utilizing a phase difference based on the difference in frequency.
The phase of the magnetic resonance signals is affected by inhomogeneity of the static magnetic field strength, and imaging by a so-called Dixon method is performed to separately image water and fat without being affected by the magnetic field inhomogeneity. This technique involves obtaining an image with the phase of magnetic resonance signals from water (which will be simply referred to as a water phase hereinbelow) in phase with that of magnetic resonance signals from fat (which will be simply referred to as a fat phase hereinbelow) and an image with the water phase opposite to the fat phase, and generate a water image through the sum of these images and a fat image through the difference between these images.
However, the Dixon method requires normal imaging operations for two images to obtain one image, resulting in low efficiency. Moreover, since the phase change rate for fat decreases relative to that for water as the static magnetic field strength lowers, a longer TE (echo time) is required to obtain signals with water and fat in phase by using, for example, a gradient echo, and accordingly signal decay increases, making it impractical to employ the Dixon method in a magnetic field having a strength as low as 0.2 T, for example.
It is therefore an object of the present invention to provide a magnetic field inhomogeneity measurement method and apparatus for performing imaging with water and fat separated in an efficient manner, a phase correction method and apparatus based on the magnetic field inhomogeneity, and a magnetic resonance imaging apparatus for performing imaging with water and fat separated in an efficient manner.
In accordance with a first aspect of the invention, there is provided a magnetic field inhomogeneity measurement method comprising the steps of: obtaining a tomographic image of a subject within a static magnetic field space using magnetic resonance, the tomographic image having a phase difference of 2xcfx80/n (nxe2x89xa72) between pixel data of water and fat; multiplying the phase of pixel data of the tomographic image by n; correcting wraparound caused by the phase multiplied by n exceeding a range between xc2x1xcfx80; and multiplying the wraparound-corrected phase by 1/n to obtain a phase representing magnetic field inhomogeneity in the static magnetic field space.
In accordance with a second aspect of the invention, there is provided a magnetic field inhomogeneity measurement apparatus comprising: imaging means for obtaining a tomographic image of a subject within a static magnetic field space using magnetic resonance, the tomographic image having a phase difference of 2xcfx80/n (nxe2x89xa72) between pixel data of water and fat; phase-multiplier means for multiplying the phase of pixel data of the tomographic image by n; wraparound-correction means for correcting wraparound caused by the phase multiplied by n exceeding a range between xc2x1xcfx80; and magnetic field inhomogeneity-measurement means for multiplying the wraparound-corrected phase by 1/n to obtain a phase representing magnetic field inhomogeneity in the static magnetic field space.
In accordance with a third aspect of the invention, there is provided a phase correction method comprising the steps of: obtaining a tomographic image of a subject within a static magnetic field space using magnetic resonance, the tomographic image having a phase difference of 2xcfx80/n (nxe2x89xa72) between pixel data of water and fat; multiplying the phase of pixel data of the tomographic image by n; correcting wraparound caused by the phase multiplied by n exceeding a range between xc2x1xcfx80; multiplying the wraparound-corrected phase by 1/n to obtain a phase representing magnetic field inhomogeneity in the static magnetic field space; and correcting the phase of pixel data of the tomographic image using the phase obtained.
In accordance with a fourth aspect of the invention, there is provided a phase correction apparatus comprising: imaging means for obtaining a tomographic image of a subject within a static magnetic field space using magnetic resonance, the tomographic image having a phase difference of 2xcfx80/n (nxe2x89xa72) between pixel data of water and fat; phase-multiplier means for multiplying the phase of pixel data of the tomographic image by n; wraparound-correction means for correcting wraparound caused by the phase multiplied by n exceeding a range between xc2x1xcfx80; magnetic field inhomogeneity-measurement means for multiplying the wraparound-corrected phase by 1/n to obtain a phase representing magnetic field inhomogeneity in the static magnetic field space; and phase-correction means for correcting the phase of pixel data of the tomographic image using the phase obtained.
In accordance with a fifth aspect of the invention, there is provided a magnetic resonance imaging apparatus comprising: imaging means for obtaining a tomographic image of a subject within a static magnetic field space using magnetic resonance, the tomographic image having a phase difference of 2xcfx80/n (nxe2x89xa72) between pixel data of water and fat; phase-multiplier means for multiplying the phase of pixel data of the tomographic image by n; wraparound-correction means for correcting wraparound caused by the phase multiplied by n exceeding a range between xc2x1xcfx80; magnetic field inhomogeneity-measurement means for multiplying the wraparound-corrected phase by 1/n to obtain a phase representing magnetic field inhomogeneity in the static magnetic field space; phase-correction means for correcting the phase of pixel data of the tomographic image using the phase obtained; and image-producing means for separately producing a water image and a fat image using the phase difference in the phase-corrected pixel data.
In accordance with a sixth aspect of the invention, there is provided a magnetic resonance imaging method comprising the steps of: obtaining a tomographic image of a subject within a static magnetic field space using magnetic resonance, the tomographic image having a phase difference of 2xcfx80/n (nxe2x89xa72) between pixel data of water and fat; multiplying the phase of pixel data of the tomographic image by n; correcting wraparound caused by the phase multiplied by n exceeding a range between xc2x1xcfx80; multiplying the wraparound-corrected phase by 1/n to obtain a phase representing magnetic field inhomogeneity in the static magnetic field space; correcting the phase of pixel data of the tomographic image using the phase obtained; and separately producing a water image and a fat image using the phase difference in the phase-corrected pixel data.
(Effect)
According to the present invention, water and fat are made in phase by multiplying the phase of pixel data by n, and wraparound that exceeds a range between xc2x1xcfx80 is corrected and then is multiplied by 1/n to obtain a phase distribution corresponding to magnetic field inhomogeneity. The phase distribution is used to correct the phase of pixel data, thereby eliminating the effect of the magnetic field inhomogeneity. Using the pixel data from which the effect of the magnetic field inhomogeneity has been eliminated, separate water and fat images are provided using the phase difference.
Thus, the present invention can provide a magnetic field inhomogeneity measurement method and apparatus for performing imaging with water and fat separated in an efficient manner, a phase correction method and apparatus based on the magnetic field inhomogeneity, and a magnetic resonance imaging apparatus for performing imaging with water and fat separated in an efficient manner.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.